An AC power source produces a potential difference between its output terminals. Conventionally, the potential of the terminals is measured with respect to electrical ground. In typical power distribution systems, the AC frequency is 60 Hz.
To ensure personal safety and prevent property damage, overcurrent protection is provided on the AC line, particularly on high voltage lines. For example, a circuit breaker may be placed in a line to interrupt the electrical current in the line if the current exceeds some predetermined setting. A circuit breaker is an electrically-actuated switch that opens to stop current flow. Opening of the switch, also referred to as "tripping", may be in response to thermal or electromagnetic conditions in the circuit. After the circuit breaker opens, it may be reset or closed to allow current to again flow through the line.